Abstract 1936: A New Subtype of Human Delta-Storage Pool Deficiency is Associated with a Selective Defect in Platelet Multidrug Resistance Protein 4 (MRP4/ABCC4) Expression
Background & Aim: Delta storage pool deficiencies (delta-SPDs) are among the most frequent platelet disorders. Many delta-SPDs are caused by defects in forming membranes of the granules. However, ADP concentration in delta granules is so high that there must be an active transport protein in the granule membrane. We have shown previously that the amphiphilic anion transporter MRP4 (ABCC4) is highly expressed in human platelet delta-granule membranes and provided evidence for its involvement in ADP storage. We now present a new subtype of human delta-SPD with normally expressed delta granules but selective adenine nucleotide deficiency.
Methods & Results: We studied platelet function and MRP4 expression in two patients which exhibit easy bruising and nose bleeding and typical functional patterns of delta-SPD in platelet aggregation studies. The delta granules had selectively reduced adenine nucleotide contents, while their serotonin concentration was normal. MRP4 expression was diminished in platelets of both patients as indicated by immunoblotting with two antibodies and by immunofluorescence microscopy. Expression and localization of alpha- and dense-granule marker proteins were not significantly altered. In erythrocytes and lymphocytes from these patients, however, MRP4 expression was detected. Sequencing of the coding region of the MRP4 gene of these patients did not show mutations explaining the reduced expression. For comparison, we analysed platelets from “classic” delta-SPD patients with also low platelet serotonin levels. Here, MRP4 was well detected however localization was shifted to patches at the plasma membrane.
Conclusions: We describe the first selective (partial) platelet delta-SPD in humans caused by selective absence of a membrane transporter, distinguishable from “classic” delta-SPD exhibiting normal expression but altered trafficking of MRP4. These results support the essential role of MRP4 in platelet ADP storage and implicate that expression of this protein requires different co-expression of proteins in platelets and other blood cells.